U-series Nuclides & Particles U.S. GEOTRACES Atlantic Implementation Workshop Sept Woods Hole Oceanographic Institution Bob Anderson

Selected topics 230 Th - Paleo flux proxy? 231 Pa ( 231 Pa/ 230 Th ratios) - Tracer of ocean physics or particle chemistry? Particle Transport and Internal Cycling of TEIs

Part I: 230 Th Uniform well defined source (U decay) Potential tracer for overturning circulation (Marchal) Tracer for scavenging processes “Constant Flux Proxy” for paleoceanography

Sources Pict file

Boundary pdf file Principles: If  scav <  mix, then tracer removal rate equals production. If  scav >  mix, then tracer removal increases with particle flux.

230 Th-normalized Flux Calculation In sediments: The preserved flux (F) of a sedimentary constituent (i) can be estimated by measuring concentration ratios, without requiring a detailed age model.

The “Problem” - Contrasting views of Equatorial Pacific Paleoproductivity Marcantonio et al., 2001

230 Th Skeptics

Issues raised by skeptics (These apply to other TEIs as well) 1)Does 230 Th “leak out of the bottom”? (is it released back to the water column during regeneration of biogenic particles at the sea bed, thereby violating the assumption that F Th = P Th ?) 2)Is scavenging of 230 Th enhanced significantly in areas of high biological productivity (high particle flux and scavenging intensity)? 3) Are particles carrying 230 Th fractionated from other phases of interest during lateral redistribution of sediments?

1) Regeneration at the sea floor? Scavenging models predict [ 230 Th] to increase linearly with depth. Given the short residence time of Th in seawater, if 50% of the F Th reaching the sea bed were regenerated, then one would find strong near-bottom concentration gradients. Near-bottom profiles with sufficient resolution have never been collected! Francois et al., 2007

Ventilation or bottom scavenging? BUT - Profiles of [ 230 Th] actually decrease toward the bottom. Attributed to rapid ventilation of near-bottom waters. Could be “bottom scavenging” as observed in some Pacific 210 Pb profiles (Nozaki) Francois unpublished

Benthic regeneration? Bottom Scavenging? Ventilation? (MOC) “Simple” source makes 230 Th an ideal tracer RECOMMENDATIONS: Measure a few high-resolution near-bottom profiles in contrasting environments: –W. N. Atlantic - rapidly ventilated –E. N. Atlantic - more slowly ventilated –Region of high productivity and maximum benthic regeneration

N Atlantic Productivity Gradients From Gregg et al., 2003 (g C/m 2 /yr)

2) Enhanced scavenging in areas of high productivity (particle flux)? In principle, should occur, but “to what extent”? Low sensitivity in Models Low sensitivity in sediment trap data Low sensitivity in water column data

Model simulations Siddall et al., 2008 Model suggests enhanced scavenging at equator cannot account for observed focusing factors (high accumulation rates of 230 Th) at core sites (dots) Observed 230 Th Flux/Prod. Model 230 Th Flux/Prod.

Model Limitations Global model-data comparison Siddall et al., 2008 Point by point model data comparison shows substantial disagreement, both for dissolved and particulate 230 Th. Could model be wrong about magnitude of enhanced 230 Th scavenging?

Low Sensitivity of F Th /P Th to mass flux Francois et al., 2004 Margin sites Away from margins (downslope transport), F Th /P Th varies ~30% over a 5-fold range of particle mass flux (representative of biological productivity and scavenging intensity)

Low Sensitivity of F Th /P Th to mass flux Newer data support the conclusions of Yu et al., 2001 and of Francois et al., 2004 Th_profiling.xls

Pacific Th profiles Nozaki et al., 1981 Anderson, unpublished Chase et al Reversible scavenging models predict linear profiles when production is balanced by scavenging and decay Lateral gradients in 230 Th across broad areas of the Pacific are small!

chase Lateral gradients exist across the APF But disappear when plotted on constant density surfaces Despite large gradients in productivity and particle flux

Challenge: Small gradients require precise measurements to quantify lateral fluxes Although not normalized to constant density, profiles suggest small lateral gradients in [ 230 Th]. Given its high particle reactivity, gradients in other TEIs may be small as well. High quality measurements are required!! Francois unpublished

Part II: 231 Pa/ 230 Th Ratios Uniform well defined source at constant 231 Pa/ 230 Th = activity ratio Tracer of “Boundary Scavenging” Paleo proxy for particle flux/productivity? Paleo proxy for Atlantic MOC? Need to resolve contributions of –Deepwater ventilation –Particle Flux –Particle Composition

Boundary pdf file Principles: If  scav <  mix, then tracer removal rate equals production. If  scav ≥  mix, then tracer removal increases with particle flux.

Equatorial Pacific Sediment Traps > 2000m Particulate 231 Pa/ 230 Th scales with particle flux Basis for using 231 Pa/ 230 Th as a paleoproductivity proxy

Sensitivity to Atlantic MOC Conventional view: About half the 231 Pa produced in the N Atlantic is lost by advection to the south Figure from Henderson & Anderson, RIMG 2003

Interpretation: Bermuda Rise 231 Pa/ 230 Th reflects changes in AMOC <- low 231 Pa/ 230 Th = Pa lost by advection (McManus et al., 2004) <- 231 Pa/ 230 Th = Production ratio No Pa loss; MOC “OFF” 171 citations in 4 years HE1 LGM Figure from GEOTRACES Science Plan

Fractionation during scavenging depends on particle composition Pa adsorbs preferentially to opal High productivity generally = high diatom productivity Redrawn from Chase et al. (2002) Particulate 231 Pa/ 230 Th Range Bermuda Rise data Sediment traps

N Atlantic Core Tops: High 231 Pa/ 230 Th follows high productivity SeaWiFS Spring 2000 Avg Core Top 231 Pa/ 230 Th North of 40°N 0.094±0.024 (n=15)

Conventional view: About half the 231 Pa produced in the N Atlantic is lost by advection to the south Expect dissolved 231 Pa/ 230 Th to be lower in N Atlantic than in Pacific

Dissolved 231 Pa/ 230 Th ratios in the Pacific Without deep water ventilation (MOC)… Water Depth (m) Dissolved 231 Pa/ 230 Th (AR)

Dissolved 231 Pa/ 230 Th ratios in the Pacific are NOT greater than in the Atlantic Contrary to expectations Pacific Data Anderson & Fleisher Atlantic data R. Francois unpublished Water Depth (m) Dissolved 231 Pa/ 230 Th (AR)

Recommendations: 231 Pa/ 230 Th Measure diss. & partic. 231 Pa & 230 Th across gradients in particle flux and particle composition Establish fractionation as f(particle composition) Establish lateral conc. gradients to evaluate lateral fluxes Combine with meridional sections to model sensitivity of sedimentary 231 Pa/ 230 Th to –Deepwater ventilation (AMOC) –Particle Flux –Particle Composition

Particulate Trace Metal data VERTIGO K2 and ALOHA Bishop, J.K.B.and Wood, T.J. (2008) Particulate Matter Chemistry and Dynamics in the Twilight Zone at VERTIGO ALOHA and K2 Sites. Deep Sea Research I /j.dsr and unpublished data.

VERTIGO: June-July ALOHA; July-Aug K2 Bishop and Wood (2008) Deep Sea Research I /j.dsr

Multiple Unit Large Volume in-situ Filtration System; 12 sample depths 10 m m, Main ,000L; Aux 2000L; Side arm L Bishop and Wood (2008) Deep Sea Research I /j.dsr

ALOHA K2 (1) K2 (2) Vertical gradients reflect sinking & regeneration Particulate P K2 > ALOHA Particulate Si K2 >> ALOHA

Bishop and Wood (2008) Deep Sea Research I /j.dsr Particulate Ca K2 ≅ ALOHA

Bishop and Wood (2008) Deep Sea Research I /j.dsr Particulate Ba Higher values at K2 reflect higher productivity Particulate Mn Higher values at K2 reflect margin sediment source

P: Bishop and Wood (2008); Cd: Bishop (unpublished) 1-51 µm P:Cd > 51 µm P:Cd N Pac Trend is ~ 3000:1 dissolved Variable Cd/P ratio - Paleo proxy implications Unpublished Cd data have been removed. At Station K2, the <1µm and 1-51 µm fraction show strong surface enrichment in particulate Cd, while at Aloha there no comparable surface enrichment. Thus, the two sites have very different particulate P/Cd ratios. At K2, the particulate P/Cd ratio is less than the average dissolved P/Cd ratio whereas at Aloha the particulate P/Cd ratio is greater than the average dissolved ratio:

Si: Bishop and Wood (2008); Zn: Bishop (unpublished) Unpublished Zn data have been removed. At Station K2, the <1µm and 1-51 µm fraction show strong surface enrichment in particulate Zn, while at Aloha there no comparable surface enrichment. In general, there are higher surface concentrations of particulate Zn and larger vertical gradients at K2 than at Aloha.

Zn, Co: Bishop (unpublished) Unpublished Co and Zn data have been removed. In contrast to Zn, where surface particulate concentrations are much greater at K2 than at Aloha, concentrations of particulate Co are similar at the two stations. At Aloha, concentrations of particulate Co decrease downward from the surface to about 300 m, and then increase again. This may reflect association of Co with Mn oxides.

Recommendations: Particles Measure concentrations of major particulate phases (Ca, Si, Al, POC) to assess –Scavenging affinity of each phase for individual dissolved TEIs –Effect of scavenging intensity (particle concentration) on distributions of dissolved TEIs Measure concentrations of particulate TEIs to assess –Affinity for major phases –Associations among TEIs (e.g., Co & Mn) –Fractionation among TEIs (e.g., Pa vs Th; Cd vs. P) Combine vertical profiles of particulate TEIs with inverse models of dissolved TEIs to constrain rates of internal cycling

Black Bkg